[Broken_Hippo]

Sure, personal plans have unlimited calling. But a call center? That's multiple phone lines at business rates, and often an internal phone system as well. These are likely special, more expensive lines since they aren't receiving outside calls. I'm going to guess there are international agreements that allow for some of the cost of international connection to be paid by the caller as well (yet another way to make money: International calls).

On the other end, if they were to try to stop spam calls, it would cost money. You'd have to have a way to find out if a caller has legitimate business or not: A collection agency might be marked as spam quite a bit, yet shutting them down would likely mean a lawsuit. Sales calls seem to fall into a weird grey area. There would need to be a system for consumers to report spam calls. These all cost money to build and maintain.

Not to mention enforcement, which would be especially tricky given international calls and the bit about legitimate businesses being marked as spam.